1. Field of the Invention
The present invention relates to a connector for steel frame structures and more particularly, to a die cast connector for use in steel frame structures to effect a connection between a beam and a column.
2. Description of the Related Art
Steel frame structures such as buildings and the like are typically constructed either with welded connections or bolted fittings between beams and columns to achieve an assembly capable of bracing structures against lateral loads, such as earthquakes, wind, etc.
According to one construction technique, beams are welded to columns. A plate is welded to a column and a beam is welded to the plate. While welded connections between beams and columns are typically satisfactory under most normal loading conditions, these connections may be inadequate when the structure is subjected to abnormal loading conditions such as those that occur during earthquakes or other catastrophic events. High tensile forces placed on a weld between a beam and a column during earthquakes are often sufficient to fracture or otherwise damage the welded connection between beam and column. Thus, the welded connections tend to be the point of structural failure in welded steel frame structures.
An alternative to a welded connection is a semi-rigid connection employing a bolted fitting. One type of bolted fitting for connecting columns and beams which has been used in place of a welded connection employs a T-shaped fitting to provide a bolted connection between the beam and the column. This type of T-shaped connection includes a base plate (top of T) which is bolted to the column and a protrusion which is bolted to the flange of an I-beam by high strength bolts. In connecting a beam to a column, two T-shaped fittings are generally attached by the protrusion to the top and bottom flanges of the I-beam. The base plate of the T-shaped fitting is attached by bolts to the column. However, the bolts which connect the base plate of the fitting to the column may tend to experience a lever effect when the flange of the beam to which the protrusion is bolted is subjected to a tensile force. This lever effect or prying action places a high stress on the bolts. Therefore, various attempts have been made to strengthen these bolted connections. The fittings themselves have been strengthened to such a degree that structures employing these types of fittings tend to have failures which occur in the beams at the location where the connectors end several feet from the center of the column.
Examples of T-shaped fittings are illustrated in U.S. Pat. Nos. 3,960,458, 3,938,297, 4,014,089, and 4,074,947.
Both the known welded and known bolted connections attempt to protect the columns and joints while promoting failure in the beams rather than at the joints. This practice has resulted in solutions which reduce the economy of the steel frame system. The known connections promote hinging/failure in the beam several feet away from the center of the column. The hinging/failure of the beam creates a discontinuity in the beam so that the load carried by the beam is transmitted across the hinged portion of the beam in a different manner than prior to being hinged. Because this hinging occurs several feet from the center of the column, it magnifies the forces experienced by the column (from secondary moments/forces), thus increasing the size of the columns needed and thereby decreasing the economy of the structural steel frame assembly.
The column size may typically need to be further increased over and above design earthquake force requirements to maintain superior strength over the beam that is designed to remain the weak link. Because the structural steel yield and ultimate strengths of the beams can vary greatly due to structural steel milling practices and due to the availability of raw materials, ensuring that a column has superior strength over the beams requires engineering the columns using the worst case upper limit of the strength of the beams. For example, steel specified as having a 50 ksi minimum yield steel may actually have a 65 ksi yield. This increases column sizes which further reduces the economy of these known structures.
In addition, because the known structures promote failure of the beam, the cost of repairing the beams becomes a considerable expense due to the size of the beams, and the lack of adequate access to the beams once the structure has been enclosed by finishes. This reduces further the economy of the systems which promote hinging/failure of the beam.
It would be desirable to provide a connector that is designed to gradually yield under exceptional loads rather than transmit the extreme load to the beam causing the beam to yield. It would also be desirable that the connectors could be inspected and replaced according to whether the connectors showed observable signs of yielding. In addition, if the connector were designed to yield under abnormal loading, only the connector would need to be replaced, thereby reducing the cost of repairing the beam and column structure. Further, it is desirable that a connector be inexpensive to manufacture and have little impact on the overall beam and column structure.
According to one aspect of the invention, a connector for connecting a beam to a column includes a bulk head and a plate extending from the bulk head. The bulk head has a first surface for abutting a column and a second surface opposite the first surface. The first surface is smaller in area than the second surface. The bulk head also has a plurality of bores connecting the first surface to the second surface. The plate extending from the bulk head includes a tapered portion that diminishes in a width dimension as the plate extends from the second surface toward a free end.
According to yet another aspect of the invention, a connector for connecting beams to columns includes a bulk head having a first surface for abutting a column and a second surface opposite to the first surface. The second surface of the bulk head is greater in area than the first surface. A plurality of bores extend through the bulk head from the first surface to the second surface and receive bolts to connect the bulk head to the column. A plate extends from the second surface of the bulk head and includes one or more tuning holes for adjusting the yield strength of the plate.
According to a another aspect of the invention, a method for connecting a beam to a column is provided. According to the method a connector including a bulk head and a plate is provided. The bulk head has a first surface for abutting a column and second surface opposite the first surface. The bulk head also has a plurality of bores connecting the first surface to the second surface. The plate extending from the bulk head includes a tapered portion that diminishes in a width dimension as the plate extends from the second surface toward a free end. The plate of the connector is welded to the flange of the beam and the bulk head is bolted to the column.